In a nutshell, the simplest PIC chip is the PIC12C508. The PIC12C509 has twice the memory but both are only programmable ONCE. This type of chip is now HISTORY. We don't use them any more. The replacement is the PIC12F629. If you want analogue inputs, the chip to use is the PIC12F675, None of these chip has a low-voltage programming feature. For low-voltage in-circuit serial programming use PIC16F628. The PIC12F629 comes in: 8-pin             8-pin SMD (through        (surface  mount) hole)             PIC12F629-I/SN  8/PDIP           8/SOIC  package    SOIC = SOP = SMD IC = 050 (1.27mm) spacing. SOIC - pins are bent slightly at PC board SOP - pins are flat when touching PC board

Before we start:
Although the PIC12F629 is an up-grade of the PIC12C508 and '509, the program from a PIC12C508 cannot be transferred directly to a PIC12F629 as there are a number of differences between the two chips. The following items cover some of the differences.
1. The CALL Instruction
The CALL instruction  for a PIC12C508,9 only works to the first 256 locations.
The  CALL instruction for a PIC12F629 accesses ALL MEMORY.
2. The STACK
The PIC12C508A has only a 2-CALL stack.
The PIC12F629 has an 8-CALL stack. (8-level stack).
3. The In/Out Port
The IN/OUT port on a PIC12F629 is file 05h. This port corresponds to pins 7, 6, 5, 4, 3, and 2 for GP0, GP1, GP2, GP3 GP4 and GP5.
Remember: GPIO,3 (GP3) (pin 4) is an INPUT-ONLY pin.
When writing a program, use GPIO,0 GPIO,1 GPIO,2 GPIO,3 GPIO,4 GPIO,5 in your program and the compiler will do the rest. (GPIO = General Purpose Input/Output).
For instance, If you want to SET (make HIGH) pin 7, you must do two things:
1. Make sure GPIO,0 is an OUTPUT bit by making the corresponding bit in the TRISIO register "0."  This is done via the following instructions: Remember: "0" = output,  "1" = input

    bsf         status, rp0      ;bank 1
    movlw     b'xxxxxxx0'    ;(x= don't care = does not matter)
    movwf    TRISIO           ;the lowest bit will be an OUTPUT
    bcf         status, rp0     ;bank 0

2. Make the lowest bit of the output port = 1 (HIGH). This is done via the following instruction:

    bsf      GPIO,0

Do not set more than one bit at a time in the GPIO (in/out) port. In other words, do not create two instructions such as:

      bsf      GPIO,0
      bsf      GPIO,2

If you need to clear or set two or more output lines, perform the operation via a single instruction, thus:

    movlw   b'xxxxx1x1'
    movwf   GPIO

You need to know the state of all the outputs and include this in the value you are going to load into GPIO via movlw b'xxxxxxxx' etc. 

As can be seen from the above, the six lower bits of file 05 are connected to 6 pins of the chip to connect the microprocessor to the outside world.  This file is like all the other files (from 20h to 5F - 64 files) as it can be operated-upon (incremented, decremented, shifted left or right, plus the other operations). The only difference is the contents of file 05 can be exported to the outside world and the outside world can influence the file. When any of the bits of file 05 are configured as "out," the value of the bit will make the corresponding pin of the chip either HIGH or LOW. When it is "set" (=1), the pin will be HIGH. When it is "clear" (=0), the pin will be LOW.   


READING AN INPUT LINE
When any of the bits of file 05 are configured as "input," (this is done via the TRISIO instruction) the HIGH or LOW on the pin of the chip (this HIGH or LOW will be created by something in the outside world making the line HIGH or LOW) will be read by the micro as a HIGH or LOW, (when the file is read via an instruction such as btfss GPIO,1 or btfsc GPIO,1 - bit test1 in file 05h, skip if set or bit test1 file 05h, skip if clear).
This "reading process" can also be done when the contents of file 05 (GPIO) is moved to W.  The instruction to do this is movf  05h,0  This instruction tells the micro to move the in/out port to the working register. The working register is called "W" and has the destination "0" - a file has the destination "1." The contents can also be shifted, incremented, plus other instructions.
Here are some instructions to read the input bit:
In most cases, the first bit (or line) to use in a program is pin 4 as this line is INPUT ONLY. It corresponds to GPIO,3.
Using the instructions from above, we have GPIO,3 as an INPUT and all other lines are OUTPUT.
We are looking for a HIGH on the input line.
To read the bit, use the following instructions:

    btfsc    GPIO,3    ;This will test the input bit and if it is LOW, the micro goes to movlw xx
    GOTO  PhaseA   ;This instruction takes the micro to a new sub-routine
    movlw  xx

If you don't know the state of some of the bits (or don't want to alter them - by mistake), you can use the XOR instruction.
For example, to turn ON bits 0, 1 and 2, the instructions can be:
bsf  GPIO,0     b'00000001'
bsf  GPIO,1     b'00000010'   
bsf  GPIO,2     b'00000100'
But this will result in only the third instruction being carried out. We mentioned above, not to use multiple bit-setting as it will fail to work.
The answer is to use the XOR instruction
Combine the 3 instructions to get: b'00000111'
movlw    07h
xorwf    GPIO
Only the three lowest outputs will go HIGH.
To turn OFF the three lowest outputs, repeat the same instructions:
movlw    07h
xorwf    GPIO
Only the three lowest outputs will go LOW.
Actually, the bits will TOGGLE.

The in/out port (file 05h for PIC12F629) is shown in RED in the following map. It is called GPIO:

Voltage on an Input line
Most input lines are classified as TTL and the voltage must be above 2v (for 5v rail) for the chip to detect a HIGH.  GP2 is Schmitt Trigger input a it TMR0 clock input. This line requires 2.5v to detect a HIGH and 1.75v to detect a LOW. The TTL input have a very small gap between HIGH and LOW and to make sure the input line detects correctly, the input voltage must be much higher or much lower than the detection-point.

4. Reading the Input Port
To read the individual pins of the input port of a PIC12F629, use one of the following instructions. (GPIO,3 (GP3) (pin 4) is an INPUT-ONLY pin and this pin is the first to use.
If the input pin is normally low and goes HIGH when a signal is sent to the micro, use one of the following:
    btfsc 05h,0   (This the same as:   btfsc    GPIO,0)
    btfsc 05h,1
    btfsc 05h,2
    btfsc 05h,3   - this is pin 4 - GPIO,3  - use first in your program
    btfsc 05h,4
    btfsc 05h,5
The next instruction will be:
    goto  task1    - input has been detected

It may be difficult to read the same input bit further down a program.
This is solved by setting and clearing the bit, thus:

         bsf      status, rp0       ;Bank 1
         bcf      TRISIO,0
         nop
         bsf      TRISIO,0 
         bcf      status, rp0       ;bank 0

5. The General Purpose Files - called GPR's (General Purpose Registers)
The General Purpose Files are the files you use in your program for sub-routines such as delays, comparison, incrementing, decrementing, etc. A program can start by loading a value into GPIO and outputting it to the outside world (such as a LED). A value (called a literal) is loaded into a file (a General Purpose File) and decremented until it is zero. This allows the LED to be viewed.
In the diagram above, you can see the 25 General Purpose Files in a PIC12C508/9 do not match up with the 64 files in a PIC12F629.
This means any program written for a '508 will not work in a PIC12F629.
The solution is to add 20h to the files that have been used in a '508 program to move them so they align with the PIC12F629 files.
The following table shows the result:

6. Incrementing a File
When incrementing a file such as:
   incf   2Ah,1
The result is placed in the file and the working register "W" is not affected.
With the instruction:
   incf   2Ah,0
only the working register "W" contains the result. In other words, the file is
NOT INCREMENTED!!! 
With the instruction:
   incf   2Ah
the assembler assumes ",1" and increments the file.  But it is best to always include the directive.

In the above, W=0 and f=1 where W is the working register and f is the file being incremented. 

7. Current of an Output Line
The maximum current of an output line is 25mA. In the following diagram, no current-limiting resistors have been included and the current through the red LED will be more than 25mA as the voltage across the LED is 1.7v and theoretically the current will be as high as the battery can deliver. The chip will provide some current-limiting but the flow will be higher than 25mA and the chip may be damaged. A PIC12F629 will deliver about 33mA if a current-limiting resistor is not included, or is too-low in value.

Two 47R resistors are needed as shown in the schematic below:

8. Driving a LOAD
The output current of each line of a microcontroller is very small. It is only 25mA. The only two items that can be driven directly from an output are a LED and a mini piezo diaphragm.
The following diagram shows how these items can be connected (interfaced).

For a current greater than 25mA, a transistor will be needed. A transistor will be able to deliver about 500mA to 1Amp. This is assuming the transistor has a gain of 20 - 40 and this about the capability of a transistor - called a small-signal transistor.
For larger currents, a MOSFET will be needed. These devices will deliver 20 amps or more to a load.
There are two ways to describe the requirement of "driving a load."
You can say you want to "drive the transistor as an amplifier" or "drive the transistor as a switch."
When a transistor is used in an analogue situation, there is a big difference between these two statements.
In an audio amplifier, when the transistor is amplifying the signal, the output is a faithful reproduction of the input, but with a larger amplitude. Or the shape of the signal will be identical but with a larger current capability.
But if the transistor in an audio amplifier is driven as a switch, the output will rise very high as soon as the input waveform starts to rise. In most audio cases this will create distortion.
But with a microcontroller we are dealing with a digital circuit and the output is either 0v or rail volts (5v).
Any transistor connected to the microcontroller can only have two states. When the output is zero, the transistor is turned off. This is called CUTOFF. When the output is high, the transistor is turned ON fully. This is called SATURATION.
These two states are called SWITCHING STATES. When a transistor is in either of these two states, it dissipates the least amount of heat. When a transistor is CUTOFF it is obvious that the heat dissipation is low as it is zero.
When a transistor is SATURATED, it has the lowest voltage across it and although the current flow is the highest, the heat dissipated is the lowest as the heat loss is multiplication of current and voltage.
Suppose we want to drive a wailing siren from a microcontroller. If the microcontroller delivers the varying tone to the speaker, we say the transistor is acting as an amplifier. It is acting as a DIGITAL AMPLIFIER. It is being switched ON and OFF at different rates and the result is a wailing sound.  
When we refer to the transistor as a SWITCH, we often require it to turn on a device for a long period of time - such as a relay or LED.

8a. Driving a LOAD - circuit TRICK:
You can increase the current to a pair of Infra-red LEDs from 25mA to 39mA by adding the 120R resistor.
To do this there are a number of calculations to remember:
The voltage across an IR LED is about 1.5v to 1.7v and an IR LED can be driven at a higher current than an ordinary LED.
Some IR LEDs accept 100mA or more for a very short pulsed duration and this arrangement will produce 39mA.
When the drive-line is HIGH, it delivers 25mA and the 120R delivers 13mA, making a total of 39mA.
When the drive-line is LOW, it sinks the current through the 68R and 120R (in series) and this is 25mA. The voltage across the two IR LEDs is 1.8v and this is below the "turn-ON" voltage for the two LEDs.

Increasing the output current

9. GP3 - pin 4    Driving a LOAD
GP3 is an INPUT Only and cannot be used to drive a load.  
The only drive-lines for PIC12f series are:
GP 0 -   pin 7   
GP 1  -  pin 6   
GP 2  -  pin 5  
GP 4 -   pin 3 
GP 5  -  pin 2

10. 100n across the micro
Don't forget the 100n across the power rails. Place it near the micro.  It stops the micro starting-up half-way through the program! Without the capacitor, the micro can start anywhere down the program and create all sorts of unusual effects.


11. The Hysteresis
The input lines of a PIC chip need about 5v to detect a HIGH and about 0v to detect a LOW.
But when the voltage is about 2.5v, it will register a HIGH at about 2.01v and a low at about 1.99v.
This means the gap between a HIGH and LOW is very small and this gap is called  the HYSTERESIS.


SOME APPLICATIONS:
Although the PIC12F629 is the simplest device in the PIC range, it can can carry out operations such as monitoring and controlling:
Battery Monitoring and Voltage Detection
Watchdog Timing
Circuit Resetting
Voltage Detecting

 There are many specially-designed chips for these applications, but they must be purchased in large quantities. To keep your inventory down, you can use a PIC chip with a few external components to perform the operation. All you have to do is design the circuit (the detection section) and program the chip.

READING A PIC CHIP
If you have a PIC12C508/9 chip and want to get the program and convert it to suit a PIC12F629, you will need to fit it into the socket of our Multi Chip Programmer and read it via ICProg.
To do this you will need to pick PIC12C508 from the list of microprocessors from the "drop down" menu. You will also need disasm.dll or the Disassembler link above.
For disasm.dll, use the "lightening" symbol to import the code from the chip. It will appear in the frame on the screen. Use the "A" button (next to the "H" button) to see the disassembled program.
You can then copy and save the program in Notepad.  Only chips that are not Code Protected, can be read. We only have some information on reading chips that are Code Protected: Cracking a PIC Chip
If you have the .hex file, you can get the assembly file via disasm.zip as explained above.
Once you have the assembly file, you can change the files as shown above.
 
SOME PROGRAMMING TRICKS:
The author has found problems with some instructions.
The PIC12F629 does not like setting a bit on the in/out port and then setting or clearing another bit in the next instruction.
The problem instructions are something like this:

    bsf   05,2
    bsf   05,4

The micro does not set the second bit when it is executing the program. The answer is to work out the value in file 05 before the two instructions, add the value of the bits and place the value in the in/out port via the following two instructions:

   movlw  xxh
   movwf  05h

Alternatively, a meaningless instruction can be inserted between the two "bsf" instructions. This MAY work but is not as guaranteed as the method above.

    bsf   05,2
    nop 
    bsf   05,4

Each new program or modification MUST be saved with a new name as either MPASM or IC_Prog will not detect the changes and you will get the old program burnt into the chip and wonder why the changes did not fix the problem!

USING A SIMULATOR AND DEBUGGER
To help developers get a program to work successfully, a number of programming tools have been created. Two of these are a SIMULATOR and a DE-BUGGER. These are software programs and a simulator runs your program and lets you know if it will work. It seems to be the answer to a dream. But a simulator has many limitations. It assumes any hardware devices connected to the program are "ideal devices."
For example, it will assume a switch has no "contact bounce" and is pressed and released very quickly. This does not happen in real life, so your program must be re-tested in actuality.
Also, any output to LEDs etc assume the items are connected around the correct way. You will still have to test the actual circuit and make sure the current is within the limits of the micro.
A de-bugger is also another piece of software that tries to locate a fault in your code.
It has a "single step mode" in which it shows the contents of each register after an operation.
Sometimes these program will assist but let me assure you that they can also lead you "down the garden path" and in the wrong direction.
I have used them and concluded that they create more frustration than assistance.
The best method is to start with a small number of instructions and make sure the program works. Add a few more instructions and re-test the program. Re-name the file each time you add instructions and slowly build up the program. This is the best way to avoid frustration.  
We have provided explanations and helpful advice in our Library of routines:  A-E   E-P   P-Z
and provided a number of programs for you to analyse and use. The main thing is not to use an instruction unless you know exactly what it does. Do not try any "clever" programming, with exotic instructions as the result can be completely different to what you expect.  
The first thing to do create a subroutine called a delay. Call it   "_200mS" for 200milliseconds delay.

_200mS     movlw      0C8h   ; equal to 200
                 movwf      fileB
AA            decfsz     fileA,1
                 goto        AA                
                 decfsz     fileB,1
                 goto        AA
                 retlw        00

Create a routine to turn a LED "on" and "off" using the delay. 

such as:

On_Off     bsf       gpio,0        ; put a LED on pin 7 with a 470R resistor.
               call      _200mS
               bcf       gpio,0
               call      _200mS
               goto    On_Off

If you want to see if the micro has advanced down a program to a particular location, put:
goto   On_Off    and the LED will blink.

SOME CIRCUIT TRICKS:
The author has found the PIC12F629 chip must be fitted to a circuit when the voltage on the rail is ZERO. If a voltage is present, some of the files in the chip will get corrupted and the program will not run correctly. For example, some of the outputs will not go HIGH when required. The remedy is to turn off the project and wait for the electrolytics to discharge. You can also remove and replace the chip if you find it is performing incorrectly.
The addition of a 10k resistor across the power rails will discharge the electrolytics to zero and help solve this problem. Also, the addition of a 100n (very close to the chip rails) will provide stability for the chip and allow the internal oscillator to work correctly and prevent outside spikes from upsetting the chip.

THE PIC12F629 - some preliminary points:
The PIC12F629 is an 8 pin chip that can be re-programmed up to 1,000 times.
It is an up-grade of the PIC12C508 and '509 as it can be re-programmed.
(The PIC12F629 does not have analogue inputs - the PIC12F675 has 4 analogue inputs.)

For more details on PIC chips, see our Chip Data Ebook.

Here is a list of the chips we suggest for the beginner to microcontroller programming.
Start with the 8-pin 12F629 if you have only 1 to 5 items to "drive" (GP3 is input only).  Or use the PIC16F628A if you want to drive up to 15 items (RA5 is input only).